1. Field of the Invention
The present inventions relates to methods of determining random sound absorption of materials and to an apparatus utilizing such methods.
2. Background Art
There is an increasing demand for the reduction of sound levels to improve perception in the passenger compartments of automobiles. Development of methods and systems that accomplish such reductions require an understanding of the potential internal and external noise sources in an automobile as well as the effect of various automobile components in masking or attenuating such noise. Moreover, reduction of noise in the 1 KHZ to 5 KHz frequency range is particularly desirable due to the increased sensitivity of vehicle passengers in that range for speech intelligibility and speech clarity.
Various testing methodologies are used to evaluate the sound absorbing properties that may be included in automobile applications. In particular, two test types are utilized by many automobile component suppliers. These two test types are provided by ASTM C384, ASTM E1050, and ASTM C423. The ASTM C384 and ASTM E1050 tests gives the sound absorption performance of a material by using sound waves that impinge perpendicular to the surface of a material being tested. The sound performance is evaluated from the measured absorption versus frequency plots determined in this method. FIG. 1 provides a schematic of an apparatus that is used to perform normal incidence sound absorption measurements in accordance with ASTM C384. Sound absorption apparatus 10 includes sound source 12 which forms a plane sound wave traveling along one direction 14 down tube 16. The sound wave is reflected back by test sample 18 to produce a standing wave. Test sample 18 is mounted on a sample holder 20. The induced standing wave is analyzed with one or more microphones 22, 24. The normal incidence sound absorption coefficient, αn, is determined from the standing wave ratio at face 24 of the test specimen. The impedance ratio, z/ρc, is also found by measuring the position of the standing wave with reference to face 26 test sample 18.
ASTM C384 is capable of measuring small samples (on the order of 100 diameter) with a tabletop apparatus. Moreover, measures are performed quickly on the order of minutes. ASTM C423 on the other hand uses sound waves that impinge on a sample with a random angle of incidence. Again, absorption versus frequency plots are obtained by this methodology. However, it take several hours to perform the measurement of ASTM C423. Moreover, the equipment used for random incidence sound absorption is large. Typically, the chamber used in these methods are about 125 cu feet with 72 square feet in surface area of about 72 square feet. In addition, the test chamber needs to be a reverberant room. Also, adding the expense of the ASTM C423 method, multiple microphones or a traversing microphones are used. The material to be tested is often introduced in a manner that introduces edge effects that make the results somewhat not repeatable. Although the normal incidence method of determining sound absorption is quick and inexpensive, there is no clear way of predicting random incidence behavior from normal incidence data.
Accordingly, there exists a need for inexpensive simpler methods and apparatus for determining random incidence sound absorption.